Why is this box checked (by default) in the Social Security worksheet?

That is just an indicator that you reported Social Security benefits on the 2024 tax return.  Information received from the 2024 tax data file that was transferred to the 2025 tax return.

This has absolutely no effect on your 2025 Social Security benefit entries.

Using the Step-by-Step interview would be more efficient.

To enter Social Security benefits reported on form SSA-1099

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Scroll down to Retirement Plans and Social Security
On Social Security (SSA-1099, RRB-1099), click the start or update button

Thanks DoninGA

I have been using TurboTax for many years. Since it was first legal to print the tax return on my Digital LA50 pin feed dot matrix printer. I always do the "Forms" because I find it more efficient. The step-by-step interview method is actually more cumbersome for me but thanks for the suggestion. 

I just hadn't noticed that checkbox before and when I read it, I couldn't figure why it was there. Perhaps it should simply say that information is/will be carried over from the previous tax year.